One approach to scarce resources:
If for example ore was only produced on 2, 10, & 12 then it is likely to be in short supply. Therefore you will want to try to ensure that you produce some if you can by placing one of your initial settlements next to that 10 hex. Otherwise it will be expensive to trade for the ore via other players, the ports or the bank.
If in doubt (or because you're placing early) it is generally a good idea to place the road pointing towards the edge of the board as other players are less likely to build on the coast and block you in.
You can place your settlements in such a way that they block your opponents. Since settlements must be at least two sides apart, you can easily block future placement options for your opponents. Instead of taking the highest probability intersection and hoping to get to the next highest one two sides away, you might decide to place your settlement in the middle of those two and take command of all three of those intersections.
While I have advocated probability analysis, Octavian's approach seems a little overboard. It's hard to argue with his logic. I'll try by saying, in the games I've played, there's usually more competition for the lumber and brick than the ore and grain. And maybe the increased production from cities makes up for the extra resources needed. But my guesses aren't very scientific.
Next up from the BGG site: Thoughts on trade.